Generally, as a heat sealable film used for packaging, a coextruded polypropylene resin laminate film wherein a polyolefin resin having a low melting point is laminated on a polypropylene resin, and a polypropylene resin laminate film, wherein an unoriented polyethylene resin film or polypropylene resin film and an oriented polypropylene resin film are laminated, have been frequently used.
The above-mentioned coextruded polypropylene resin laminate film has a certain degree of heat sealing strength, but insufficient to stand packaging of heavy goods such as cut vegetables for commercial use. For packaging of heavy goods, therefore, a polypropylene resin laminate film, wherein an unoriented polyethylene resin film or polypropylene resin film and an oriented polypropylene resin film are laminated is used. While these laminate films have sufficient heat sealing strength, however, they require a laminate step using an organic solvent and the like, which is economically unpreferable from the aspects of influence on the global environment.
Therefore, improvement of the heat sealing strength of the above-mentioned coextruded polypropylene resin laminate film has been variously considered. For example, patent reference 1 shows an embodiment using a polypropylene resin as a substrate layer and a linear low density polyethylene resin as a heat sealable layer, and patent reference 2 proposes an embodiment wherein an adhesion layer is formed between a polypropylene resin layer and a linear low-density polyethylene resin layer. In addition, patent reference 3 has reported that sufficient heat sealing strength can be obtained by defining the MFR (melt flow rate) of the resins to be used for respective layers.
[patent reference 1] JP-A-9-207294
[patent reference 2] JP-A-10-76618
[patent reference 3] JP-A-2003-225979